Nowadays, transmission techniques of telecommunication networks are being gradually evolved from the TDM-based circuit switching mode to the IP-based packet switching mode. In the traditional TDM networking scheme, it is very easy for downlink nodes of a link to restore a synchronization signal of uplink nodes from the E1/T1 link of TDM. However, when the IP networking scheme is used, transmission of synchronous information of the physical layer is not supported due to asynchronous transmission characteristics of IP networks. Several existing network synchronization schemes include Global Positioning System (GPS), Ethernet Synchronization (ITU-T G 8261), Precise Time Synchronization Protocol (IEEE 1588), and the like, wherein the IEEE 1588 protocol has no demand for network load, can simultaneously transmit frequency and phase information, is with the identification of time stamps located at the underlying hardware, and is high in restoration precision of synchronous information, thus becoming the ideal network synchronization scheme.
The network synchronization principle of IEEE 1588 is based on such an assumption that uplink and downlink delays of the communication link are symmetrical. The main process of the existing synchronization method of IEEE 1588 is to obtain a time offset value between a master clock and a slave clock from the times when messages are transceived between the master clock and the slave clock, and to synchronize the time of the master clock and the slave clock according to the time offset value.
The inventors found at least the following problems that existed in the prior art during the process of realizing the present invention: asymmetry of operating modes (such as modulating mode and the like) of opposite ends or nonequivalence of distances between incoming and outgoing physical links (such as optical fibers and the like) would cause asymmetry of unlink and downlink delays—taking the xDSL line shown in FIG. 1 for example, where the uplink has a bandwidth of 1.5 Mbps, whereas the downlink has a bandwidth of 512 Kbps, and the nonequivalence of the uplink and downlink bandwidths leads to asymmetry of the link. Once asymmetry of link delays occurs, synchronization precision of the network will be severely reduced.